1. Field of the Invention
The disclosure involves a formula, mixing procedure, etching technique and application of an etchant for revealing defects in a Type II superlattice matched to (100) GaSb.
2. Description of Related Art
One aspect of this disclosure involves a method for revealing crystallographic defects on the (100) face of Type II InAs/GaInSb based superlattice structures by preferential wet chemical etching.
Type II InAs/Ga1-xInxSb superlattices (T2SLs) lattice matched to GaSb substrates (x typically≦20%) are an important semiconductor heterostructure material systems for applications in mid-, long- and very long-wave infrared detectors. They have theoretical performance limits well beyond those of incumbent technologies based on InSb and HgCdTe.
T2SLs were first introduced in the 1970s by Sai-Halasz, Tsu and Esaki, and then a decade later, were proposed by Smith and Mailhiot to be used for infrared detection. Presently, state-of-the-art T2SL material is grown using molecular beam epitaxy (MBE) on (100) GaSb substrates.
The period structure of the basic binary form of T2SL consists of a pair of 1 to 5 nm-thick InAs and GaSb layers. Approximately one additional monolayer (ML) of InSb (α=6.4794 Å) is also required in each period to balance the strain with respect to the GaSb substrate (α=6.0959 Å) resulting from the shorter lattice constant of InAs (α=6.0583 Å).
The ternary T2SL is an alternate form in which GaSb is replaced by Ga(1-x)In(x)Sb in the superlattice, and strain is balanced by adjusting the InSb-alloy fraction “x” in the ternary layer and dispensing with the half-ML of InSb at each interface. These and several other types of T2SLs may be readily doped n- or p-type, and have been used to construct an enormous variety of infrared sensor structures, with epitaxial thicknesses of up to ˜15 μm's.
Despite more than 30 years of study, however, T2SL technology has not yet achieved its full theoretical promise, largely due to the presence of bulk and surface crystallographic defects generated during MBE growth and device fabrication that promote excess dark current.
Progress in understanding the nature of these defects and how to remediate them has been hindered by the difficulty in identifying specific defect structures in this material system.
One widely used technique that has been used successfully to identify defects with densities of up to ˜105 cm−2 on substrates and epitaxial material, is preferential chemical etching. Here, defect structures are identified by subjecting the material to a wet chemical etch that has the characteristic of being amplified in the presence of crystallographic defects, and then analyzing the density, location and physical structure of resulting etch pits.
Though a number of techniques exist to preferentially etch GaSb and InAs on different crystallographic planes, no such techniques have heretofore existed for superlattices composed of periodic combinations of thin layers of these materials. This is due to the fact that though InAs and GaInSb belong to the same −43 m point symmetry group, the difference in chemical composition yields a large contrast in preferential etch rates of these materials.
For etch pit defect delineation in (100) GaSb for example, a two-step etch process was used by J. Doershel, and U. Geissler where the surface was first chemically polished in a 2:18:40 (volume ratio) solution of HF:HNO3:CH3COOH, followed by etching in 2:1 solution of HCl:H2O2. Constant vigorous agitation was necessary to obtain reliable results.
Costa et al. used a (5:1) solution of H2SO4:H2O2 as well as (5:1) CrO3 (5 M aqua. solution.):HF for etch pit delineation on GaSb (100). For preferential etching of InAs (111) orientation, Yonenaga employed a 2.4×10−3 molar solution of AgNO3 in (3:2:5) HNO3:HF:H2O.
We have found no previously published data, however, on preferential etching of InAs/GaInSb superlattices, indicating the lack of a common etching solution with similar preferential etch rates for both compound materials.
Thus we submit this disclosure of a single etchant which reveals and distinguishes crystallographic defects on (100) T2SLs through etch pit structures defined by preferential wet chemical etching.